As previously mentioned, the easiest thing to do is two do this in two separate sheets - either stacked in a vertical container or with the KPI metric floating.

However, if you want it in one sheet, you could use a combination of WINDOW and RANK calcs to achieve this. What I've done is identify the 4th month in the view and used this to plot the KPI metric. It's places at 2 x the max value in the view (so most sales in a month x 2):